Insert injection molding is a molding method wherein an insert member having a preformed and predetermined shape is positioned within a mold. A thermoplastic resin is then injected into the mold to form a composite molded article comprised of the insert member and the mass of injection molded thermoplastic resin. Such a method has been conventionally employed as a means to form composite injection molded article whereby the insert member is mainly made of metal or resin.
In general, however, conventional composite articles thus prepared are not always satisfactory with respect to the airtightness, adhesion and adhesive properties at the boundary between the insert member and the surrounding resin mass. Therefore, conventional molded composite articles cannot always be used for applications where airtightness is required. This tendency is particularly apparent in the case of crystalline resins, such as polyacetal, polyester, polyamide, and polyphenylene sulfide resins. In general, such polycrystalline resins have a high rate of solidification and exhibit not only a rapid change in the modulus of elasticity around the solidification temperature, but also large mold shrinkage, which is frequently problematic. Amorphous resins more or less exhibit the above-described tendency as well, and thus cannot reliably be used in applications where high airtightness, adhesion and adhesive properties are required.
Known means for solving the problems mentioned above during the insert molding include using an insert member having a surface covered with synthetic rubber or the like. For example, Japanese Patent Laid-Open No. 42251/1978 discloses an insert molding method wherein use is made of an insert member having an organic polymer material coating provided on its surface, and more particularly discloses coating the insert material with an epoxy resin or nylon.
Although the known techniques in the art usually improve the airtightness of the insert molded article to a considerable extent, the airtightness, adhesion and adhesive properties are still sometimes inadequate when the molded article is used under severe conditions, for example, under high pressure conditions. Furthermore, the durability of the airtightness, adhesion and adhesive properties is also usually inadequate when the molded article is used in applications where it is exposed continuously repeatedly to high or low temperatures.
It should therefore be readily apparent that significant technical obstacles exist in the preparation of a functional insert molded article having excellent airtightness and durability. Therefore, improvements in insert molding methods have been needed.
The present invention is directed in improvements in insert molded articles whereby the articles exhibit airtightness, adhesion and adhesive properties at the interface between the insert member and the surrounding mass of injection molded resin. More specifically, the present invention relates to the discovery that providing a coating containing a specific component (to be discussed below) on the surface of an insert member is effective in achieving enhanced air-tight and adhesion properties between the insert member and the resin mass.